This invention relates generally to a self-cleaning electro-inertial precipitator unit for removing particulate contaminants from a gaseous stream in which particles charged by ions are induced to migrate toward a downwardly-flowing liquid film formed on the inner surface of a collector tube, the migration resulting from the combined action of electrostatic and centrifugal forces whereby the stream may be purified in the course of its passage through the collector tube and more particularly to an arrangement adapted to maintain the wall of the collector tube free of dust in the region just above the liquid inlet slot.
Electrostatic precipitators separate contaminating particles or droplets of a semi-solid or solid nature from a gaseous stream. Such precipitators are especially helpful in removing finer particles (less than 40.mu.) which cannot be extracted by conventional filters or other particle separators. In one known form of electrostatic precipitator of the dry type, the gases to be purified are conveyed through a collector tube where the particles are charged with ions in an electrostatic field, the charged particles migrating toward the inner surface of the collector tube having an opposite charge, thereby separating the particles from the gas flowing through the tube. With continued operation of a dry precipitator, the particles accumulate on the wall of the collector tube and it becomes necessary, therefore, at fairly frequent intervals, to shut down the precipitator in order to permit removal of the agglomerated particles.
With a wet-wall precipitator of the type disclosed, for example, in the deSeversky U.S. Pat. No. 3,716,966, a uniform film of downwardly flowing water is formed on the inner surface of the collector tube, the film serving to continuously flush away the contaminants, thereby obviating the need to interrupt the operation of the precipitator.
The use of centrifugal separators or cyclonic collectors for separating dust particles and other particulate contaminants of 25.mu. or larger from a gaseous stream is well known. In order, therefore, to effectively remove both large and small particles from a gaseous stream, one may first feed the gaseous stream through a cyclonic collector or inertial dust separator stage to extract the large particles from the stream and then feed the partially purified stream through an electrostatic precipitator stage to extract the fine particles therefrom as well as those larger particles not extracted in the preceding stage.
Thus U.S. Pat. No. 3,315,445 to deSeversky discloses a pollution control system in which gas scrubber and wet electrostatic precipitator stages are intercoupled in cascade relation so as to remove the full spectrum of contaminants from the stream. The practical drawback to the deSeversky arrangement, apart from the relatively high cost of providing both a gas scrubber and a wet electrostatic precipitator, is that these two units occupy a substantial amount of space. This creates installation difficulties in those installations where space is at a premium.
In the above-identified related patent applications of Reif, there is disclosed an electro-inertial wet-wall precipitator unit in which both fine and coarse particles are extracted from a contaminated gaseous stream by the combined action of centrifugal and electrostatic forces. The advantage of the apparatus disclosed in the prior applications is that it carries out in a single compact, integrated unit, functions heretofore requiring at least two units.
In the electro-inertial precipitator disclosed in the prior applications, the gaseous stream to be purified is fed at high velocity tangentially into an upper inlet section of a vertical collector tube to impart a cyclonic or swirling motion thereto, thereby causing the gas to flow in a helical path down the tube along a downwardly-flowing water film to impose an inertial force which imparts a swirling motion thereto serving to maintain the film against the tube surface.
Supported coaxially within the collector tube is a discharge electrode, a high voltage being impressed between the electrode and the water film to create an electrostatic field therebetween, the ions generated by the discharge electrode charging the particles carried by the gas. The centrifugal force created by the swirling motion of the gas induces the particles conveyed thereby to migrate toward the water film. This migration is further promoted by the action of the electrostatic field which causes the charged particles to travel toward the oppositely-charged water film. As a consequence, both coarse and fine particles are extracted from the gas and captured by the water film which washes the particles into the sump below the outlet section of the tube.
In operation, there is a tendency for water flowing through the inlet slot to wick upwardly along the surface of the upper lip of the slot, thereby wetting this surface. As a consequence, dust particles in this region which impinge on the wet surface are caused to adhere thereto to create a layer of wet dust acting as a ground electrode which accelerates further deposition and further wicking. In time, a cake of dust is developed at the upstream side of the water inlet in the precipitator tube. This cake may slightly overlap the inlet slot at various points, thereby somewhat impeding water flow and disturbing the uniformity of the water film. Moreover, these cakes occasionally break off and deposit on the wet wall of the collector tube at sites where they are difficult to wash away. Such occasions sometimes give rise to arcing.
In the copending application of Reif and McCrady Ser. No. 339,711 filed Jan. 15, 1982 , entitled "Inlet Section for Inertial-Electrostatic Precipitator Unit", there is disclosed an electroinertial precipitator in which an annular water inlet is disposed just below the gas inlet section into which is introduced the gaseous stream to be purified. To avoid wetting the surface of the upper lip of the water inlet slot and thereby causing dust particles to deposit thereon, the inlet section is provided with a hydrophobic surface. The entire disclosure of the Reif and McCrady application is incorporated herein by reference.
This hydrophobic surface is effective for its intended purpose when the water used in the unit is ordinary tap water. But it is much less effective when the water has wetting agents dissolved therein in order to ensure wetting of the entire inner surface of the collector tube. Also the effectiveness of a hydrophobic surface is diminished if the particles being collected are readily soluble in water, for the water used in the unit is recirculated therein.